Hot melt thermoplastic adhesives have been widely used in industry for adhering many types of products, and are particularly useful in applications where quick setting time is advantageous. One application for hot melt adhesive which has been of considerable interest in recent years is the bonding of non-woven fibrous material to a polyurethane substrate in articles such as disposable diapers, incontinence pads and similar articles.
In applications of this type, dispensing devices have been utilized which form the hot melt adhesive in an elongated, thin strand or fiber which is deposited atop the non-woven material. Such dispensing devices typically include a nozzle formed with an adhesive discharge opening and one or more air jet orifices through which a jet of air is ejected. A bead of adhesive is extruded from the adhesive discharge opening in the nozzle which is then impinged by the air jets to attenuate or stretch the adhesive bead forming a thin fiber for deposition onto the substrate. Examples of dispensing devices which are capable of dispensing a viscous material in the form of an elongated strand or fiber are disclosed in U.S. Pat. Nos. 2,626,424 to Hawthorne, Jr.; U.S. Pat. No. 3,152,923 to Marshall et al; and, U.S. Pat. No. 4,185,981 to Ohsato et al.
In applications such as the formation of disposable diapers, it is important to carefully control the pattern of the adhesive fiber deposited onto the non-woven substrate in order to obtain the desired bond strength between the non-woven layer and polyurethane substrate using as little adhesive as possible. Improved control of the pattern of adhesive fibers has been obtained in dispensing devices of the type described above by impacting the adhesive bead discharged from the nozzle with air jets directed substantially tangent to the adhesive bead. The tangentially applied air jets control the motion of the elongated fiber of adhesive, and confine it in a relatively tight, or compact, spiral pattern for application onto the substrate. Examples of devices capable of forming an elongated adhesive fiber, and depositing the fiber in a controlled pattern onto a substrate, are disclosed in the '424 Hawthorne, Jr. patent and the '981 Ohsato et al patent mentioned above.
In order to produce a compact spiral spray pattern of an adhesive fiber in the dispensing devices described above, it is important to ensure that the air jets are directed tangentially relative to the bead of adhesive ejected from the nozzle area of the dispensing device. This requires accurate placement of the bores or passageways through which pressurized air is ejected, which are typically on the order of about 0.015 to 0.020 inches in diameter. The boring or drilling of passageways having such a small diameter at the appropriate angles in the nozzle and/or gun body of prior art dispensing devices is a relatively expensive and difficult machining operation.
This problem has been overcome by the nozzle attachment disclosed in U.S. Pat. No. 4,785,996, which is assigned to the same assignee as this invention. The nozzle attachment disclosed in U.S. Pat. No. 4,785,996 is adapted to mount to the nozzle of a standard adhesive gun which is formed with an adhesive discharge opening connected to an adhesive passageway in the gun body, and an air discharge opening connected to an air passageway in the gun body. The nozzle attachment is an annular plate formed with a boss extending outwardly from a first surface of the plate and a nozzle tip extending outwardly from a second surface of the plate. A throughbore is formed between the boss and nozzle tip which communicates with the adhesive discharge opening in the nozzle of the gun body when the plate is mounted to the nozzle. Heated hot melt adhesive is transmitted from the adhesive passageway in the gun body, through the adhesive discharge opening in the nozzle and then into the throughbore in the plate. The adhesive is ejected as an extruded bead through the nozzle tip of the plate toward a substrate.
The nozzle attachment of U.S. Pat. No. 4,785,996 is formed with an annular notch or groove which extends from its first surface having the boss toward the second surface formed with the nozzle tip, and is located radially outwardly from the throughbore in the plate. The annular groove is provided to assist in drilling bores in the plate through which jets of pressurized air are directed at an angle of about 30.degree., and substantially tangent to, the adhesive bead ejected from the nozzle tip. One surface of the annular groove is oriented substantially perpendicular to the axis of movement of the drill bit, i.e., at an angle of about 30.degree. relative to the first and second surfaces of the plate, and sufficient clearance is provided within the annular groove to avoid interference with the drill bit. As a result, sliding of the drill bit relative to the plate is minimized during the drilling or boring operation which helps locate the air jet bores at the desired angle in the plate.
While the nozzle attachment disclosed in U.S. Pat. No. 4,785,996 facilitates accurate drilling of the air jet bores and produces an acceptable spiral pattern of a strand or fiber of adhesive, some deficiencies have been discovered in certain applications. The annular plate is mounted to the nozzle by a threaded mounting nut, and it has been found that the mounting nut can be over-torqued when the annular plate is installed. Such over-torquing of the mounting nut urges the annular plate against the nozzle of the gun with such force that the annular plate can deflect or distort thus creating a leakage path at the interface between the annular plate and nozzle. In some instances, it has been found that hot melt adhesive entering the throughbore in the annular plate has flowed radially outwardly along this leakage path into the annular groove where the pressurized air enters the air jet bores in the annular plate. This can clog the air jet bores and thus restrict the flow of air necessary to attenuate or stretch the adhesive bead to form an elongated adhesive fiber.
In addition to overtightening of the annular plate, another problem can occur during the assembly operation. Because the annular plate and mounting nut are separate pieces, the operator must properly orient the annular plate relative to the nozzle of the gun body before securing it with the mounting nut. Occasionally, the annular plate is installed upside down, i.e., with the nozzle tip facing the nozzle and the boss facing outwardly, which ruins the nozzle tip and requires replacement of the entire annular plate.
Another potential problem with the nozzle attachment disclosed in U.S. Pat. No. 4,785,996 is that its outer or second surface having the nozzle tip is not mounted flush with the rim of the mounting nut which secures the annular plate to the nozzle of the gun body. As a result, a cavity or space is formed between the nozzle tip and the rim of the nut. Particularly when the dispenser is operated intermittently, it has been found that cut-off drool, i.e., adhesive remaining after the gun is shut off, can collect in the space or cavity between the nozzle tip and mounting nut. This cut-off drool can collect and clog the air jet bores formed in the nozzle attachment, thus inhibiting the formation of an elongated adhesive fiber. In addition, a collection of adhesive fibers within such cavity is difficult to clean.
The potential problems with the nozzle attachment disclosed in U.S. Pat. No. 4,785,996 have been addressed in a one-piece nozzle cap manufactured and sold by Nordson Corporation of Amherst, Ohio, the assignee of this invention. The nozzle cap is formed from a section of hex-shaped bar stock such that the mounting nut and annular plate are integrally formed in a single, unitary construction instead of two separate pieces as in U.S. Pat. No. 4,785,996. A bore is drilled and tapped in the hex stock to form the mounting nut portion of the nozzle cap, and the annular plate is formed where such bore terminates. A first side or surface of the annular plate is thus located within the interior of the mounting nut portion of the nozzle cap, and the opposite, second surface is flush with the end of the mounting nut portion so that there is no rim or cavity between the annular plate and mounting nut as in the U.S. Pat. No. 4,785,996 described above.
The one-piece nozzle cap therefore eliminates the collection of adhesive at the outer surface of the annular plate, and prevents installation of the annular plate upside down, which are potential problems with the nozzle attachment disclosed in U.S. Pat. No. 4,785,996. Nevertheless, a number of difficulties are presented in the installation and fabrication of this one-piece nozzle cap. Although formed in one piece, the nozzle cap can be overtightened on the nozzle of the dispensing device wherein the mounting nut portion is over-torqued causing the annular plate portion to deflect or distort against the nozzle of the dispensing device. This can create the same type of leakage problems between the throughbore in the plate and the air jet bores therein described above in connection with U.S. Pat. No. 4,785,996.
With respect to the problems created during the machining operation, initially an annular groove or notch must be machined in the first surface of the annular plate to receive pressurized air for the air jet bores, and this is a difficult machining operation because access within the interior of the mounting nut portion of the nozzle cap is restricted. In fact, access is so restricted that a drill bit cannot be introduced at the proper angle within the interior of the mounting nut portion to drill the air jet bores from the high pressure or first surface of the annular plate toward the second surface. As a result, the air jet bores must be drilled from the opposite direction, i.e., from the second surface of the plate having the nozzle tip toward the first surface formed with the annular groove. While not absolutely required, an annular groove is also preferably formed in this second surface to facilitate such drilling operation. These different machining operations are performed on opposite sides of the cap which requires that it be turned over during the machining process which further adds to the time and cost of fabrication of the part.
Another problem in the machining operation of this nozzle cap is attributable to the inherent dimensional inaccuracies of hex bar stock. Such dimensional inaccuracies create difficulties in machining the air jet holes within the annular plate portion of the nozzle cap with the accuracy required to properly form elongated strands or fibers of adhesive.